Charting the Causes of Declining Rainfall

The Pacific's `El Ni phenomenon is linked to below-normal precipitation in southern Africa

METEOROLOGISTS are confident that it will soon be possible to accurately forecast the occurrence of droughts in southern Africa - and other parts of the world - by monitoring ocean temperature changes and atmospheric shifts over the Pacific Ocean.

This would allow better advance planning - stockpiling grain and coordinating targeted aid and imports - to provide for the lean years in drought-affected areas.

While the current drought in southern Africa is the worst recorded this century, it is part of a long pattern of 10-year cycles of falling and rising rainfall (see chart). The severity of the drought has been intensified by the fact that it comes after a decade of declining rainfall.

Scientists have established a direct link between the El Nino phenomenon - a periodic, rapid warming over a large portion of the tropical Pacific Ocean - and below-normal precipitation over the summer rainfall regions of southern Africa.

The drought in southern Africa is unrelated to the drought in the Horn of Africa - Ethiopia, Somalia, and southern Sudan - which is caused by degenerating conditions in the weather belt that sweeps across West Africa, through the Sahara Desert, and into the north-Central African region known as the Sahel.

"The drought that effects the Horn of Africa is a long-term drought which is self-generating with a progressive loss of vegetation and drier conditions each time," Mr. Shulze says. "There is no link between El Nino and the drought in the Horn of Africa."

The current El Nino phenomenon has dissipated, and late summer rains have fallen over substantial parts of the southern Africa region - partially breaking the drought in Zimbabwe and South Africa.

"But this does not necessarily mean that the region will have good summer rainfall," says Gerhard Schulze, director of forecasting at the South African Weather Bureau.

Whether this will be a good summer for crops, he says, depends largely on the distribution and timing of the rain. Initial rains had been on the late side and would have to be followed by good rains in January and February to sustain corn plants.

El Nino was originally the name used to refer to a warm ocean current that flows southward along Ecuador and Peru every year in December. It displaces the north-flowing and cold Peruvian current. It is usually restricted to the coastal areas and disappears by March or April.

Scientist now use El Nino to describe the rapid warming of large areas of the tropical Pacific which takes place every few years. El Nino has occurred at an average of every four years over the past three decades, ranging between two and six years apart. It leads to large energy exchanges between the ocean and the atmosphere which in turn give rise to major shifts in the atmospheric circulation.

El Nino coincides with what scientists call the Southern Oscillation - the seesaw relationship between surface pressure over the Pacific and Indian oceans.

"It was found that the pressure tends to be high over the Pacific Ocean if it is low over Indonesia and the Indian Ocean and vice versa," Mr. Schulze says.

Schulze says that of the past 23 El Nino cycles identified since 1880, 22 coincided with below-normal precipitation in summer rainfall regions. Of these, 75 percent led to droughts, while the other 25 percent led to slightly below-average rainfall.

Scientists still cannot predict the impact of the El Nino/Southern Oscillation phenomenon on weather conditions.

"But it is now possible to forecast the occurrence of a Warm Event [El Nino] or Cold Event reasonably correctly," Schulze says. "The El Nino/Southern Oscillation phenomenon - and the physical explanation of this phenomenon in particular - hold great promise for possible reliable seasonal forecasting in the future.

"There is a strong relation between the appearance - and particularly the buildup - of an El Nino event in the Pacific Ocean and lower than normal November-to-March rainfall over the summer rainfall regions," he says.

Meanwhile, the Regional Early Warning System of the Southern Africa Development Community (SADC) in Harare, Zimbabwe, monitors cloud cover by satellite to anticipate rainfall levels.

"In the late months of last year we started receiving data that the rains were dangerously late," says Roger Buckland, technical adviser to SADC's Food Security and Technical and Administrative Unit. "The satellite images showed a steady decline in the cold cloud cover, indicating that rain would be scarce."